The maternal effect developmental gene fs(1) has been studied by genetic and molecular techniques. Mutations in this locus result in homeotic transformations in the progeny of mutant females, and thus this locus represents a maternal gene that is involved in the specification of segment identity in Drosophila. The fs(1)h locus has been cloned by chromosomal walking and the region corresponding to the gene has been identified by the localization of 4 mutations within a stretch of 13 kb. Transcription mapping has shown that the fs(1)h locus is expressed in a doublet of RNAs of similar size of 7.6 kb and a 5.9 kb RNA during oocyte development; these transcripts correlate with the maternal function of the locus. A different transcript of 2.4 kb has been identified in pupae, correlating with additional functions known to be carried out by fs(1)h at this stage of development. Cloned cDNAs representing parts of the ovarian transcripts of fs(1)h have been isolated and are being sequenced. The expression of the ribosomal insertion which interrupts more than half of the rDNA in Drosophila has been studied. The DNA intercalating drugs chloroquin and ethidium bromide stimulate the transcription of the insertion, suggesting that the transcriptional repression of insertion sequences are very sensitive to the degree of torsional stress in the chromatin.